Change History
(1)

The UI isn't going to be changed simply due to personal preference/familiarity with other OSes. The minimize button isn't included because the interface is designed to eliminate the need for it, since 1) Haiku windows don't generally zoom to full screen by default, and 2) multiple workspaces let you organize them such that you don't generally need to hide them. Haiku's goal isn't generally to just do everything the same way other OSes do for the sake of it so you'd have to have a very good reason to change that.

If you really want a minimize button that badly, nothing prevents you from creating an alternative window decorator that includes one.